1. Field of the Invention
The present invention relates to novel diorganopolysiloxanes comprising pendant thioalkyl (meth)acrylate functional groups bonded via an SiC bond to the polysiloxane backbone, to a process for its preparation of such novel diorganopolysiloxanes, and to their use for the sheathing (coating) of optical fibers.
2. Description of the Prior Art
The demand in this art for organopolysiloxanes having acrylate and/or methacrylate functional groups bonded via an SiC bond to the polysiloxane backbone is greatly increasing, because the polymer crosslinks faster than the commercially available vinylated organopolysiloxanes. Such organopolysiloxanes containing acrylate and/or methacrylate functional groups are generally in the form of diorganopolysiloxane oils and can be formulated either alone or in combination with unsaturated monomers or polymers, to be subsequently crosslinked, generally under the influence of UV radiation.
Thus, U.S. Pat. No. 3,577,264 describes film-forming paint binders including such oils that can be crosslinked under the influence of radiation. U.S. Pat. No. 4,035,355 describes the crosslinking of such oils in an anaerobic medium. European Patent EP-A-016,663 describes their use as an adherent coating. British Patent GB-A-949,126 describes hydrolyzable silanes employed as adhesion promoters for reinforcing materials made of glass fibers, certain of these silanes being prepared by hydrosilylation of allyl methacrylate.
However, it is well known to this art that the hydrosilylation of compounds containing an allyl substituent in the presence of a platinum catalyst can present undesirable secondary reactions, particularly resulting in the formation of propene, unless the silicon hydride contains electron-donating groups such as chlorine atoms or carbonyl groups. In this regard, see J. L. Speier et al, J. Am. Chem. Soc. 79. 51957 (1974) and I. W. Ryam et al, J. Am. Chem. Soc., 82, 3601 (1980) and U.S. Pat. Nos. 4,503,208 and 3,767,690.
U.S. Pat. Nos. 4,011,247 and 4,554,339 describe diorganopolysiloxanes bearing both acrylyloxy units and SiH functional groups.
According to U.S. Pat. No. 4,554,339, oils which are gamma-hydroxypropylated at the end of the polymer chain are esterified with acrylic acid in the presence of an acidic earth of the montmorillonite type. This particular process presents the disadvantage of having to separate the solid catalyst upon completion of the reaction.
U.S. Pat. Nos. 4,261,875 and 4,294,974 describe a process for the preparation of diorganopolysiloxane oils having acrylate and/or methacrylate functional groups, and also hydroxyalkyl functional groups, by reacting the corresponding diorganopolysiloxane oil comprising hydroxyalkyl functional groups with acryloyl or methacryloyl chloride in the presence of triethylamine and with formation of the corresponding amine hydrochloride. The principal disadvantage of such a technique is in the necessity for removing the amine hydrochloride formed.
Another monomer which is also widely used is 2-isocyanatoethyl methacrylate (U.S. Pat. Nos. 4,543,398 and 4,600,751).
However, this monomer is sensitive to the presence of water and thus presents unavoidable toxicity hazards.
It is also possible to prepare oils containing an acrylate or methacrylate functional group by opening an epoxy (oxirane) ring using acrylic acid in the presence of a wide variety of catalysts (U.S. Pat. No. 4,293,678 and EP-A-269,114). Thus prepared are, for example, diorganopolysiloxanes comprising groups of the formula: EQU .tbd.Si(CH.sub.2).sub.3 OCH.sub.2 --CH(OH)CH.sub.2 OC(O)--CH.dbd.CH.sub.2
The oils produced are frequently very viscous because of the presence of hydroxyl groups which result in the formation of hydrogen bonds.
The reaction is also incomplete. Residual epoxy groups are often retained, and are less susceptible to UV radiation than acrylate groups. In addition, these oils display but mediocre storage stability.
U.S. Pat. No. 3,767,690 describes the preparation of organosilicon cinnamates by reacting allyl cinnamates with a silane or siloxane containing a mercapto functional group. The compounds produced crosslink under UV radiation, but much more slowly than the diorganopolysiloxanes containing acrylate or methacrylate functional groups. This process cannot be extrapolated to the preparation of diorganopolysiloxanes containing a thioalkyl acrylate functional group because it requires free-radical generators which polymerize the acrylate or methacrylate functional groups.
Furthermore, for such highly specific applications as the sheathing of optical fibers, silicone compositions are required having a high refractive index (higher than 1.48) for the primary coating of the optical fibers, and also having a high Young's modulus (higher than 100 MPa) for the secondary coating of the optical fibers.
In addition, these silicone compositions must be crosslinkable under gamma, UV, electron beam and similar radiations, in an irradiation time of less than one second.
The known diorganopolysiloxanes containing an acrylate functional group generally crosslink well in less than one second, but have a refractive index which is generally lower than 1.48 and a Young's modulus which is frequently lower than 100 MPa.